This invention relates to communication systems and in particular it relates to communication systems which make more effective use of available telephone communications circuits than systems previously known to the prior art.
Because of the extremely high cost of certain telephone communications circuits, e.g., satellite circuits and undersea transmission links, the prior art has sought various means to maximize the efficiency of existing transmission circuits. One such system is known as a time assignment speech interpolation (TASI) system. In a typical TASI system, calls from N input channels are transmitted across, for example, N/2 transmission facilities to a remote location. At that location, the N/2 facilities are again applied to N output channels. TASI systems operate on the assumption, verified as a statistical fact, that at any given time not all channels will be in use. In fact, as a general rule, channels are actively in use less than half of the time. Accordingly, TASI systems may be defined as switching systems which interconnect sender and receiver only when the sender is active, provided there is a transmission facility available at that time.
One problem associated with prior art TASI systems has been the problem of freeze-out. Freeze-out is a problem of loss of speech, or other signal which may occur whenever the number of individuals talking or starting to talk in one direction exceeds the number of available transmission facilities. The larger the number of transmission facilities, the less likely will be the chance that a particular caller will not be assigned a transmission facility when he requests one. Therefore, an attempt to interpolate two independent conversations on a single channel would result in a large percentage of speech being lost, since the statistical probability of both talkers speaking at the same time would be high. However, with a large group of facilities serving an even larger group of channels, the variation in demand for transmission facilities is much smaller as a proportion of the total capacity, and freeze-out is diminished. Thus, known TASI systems may use for example 37 or more transmission facilities to lower the statistical probability of freeze-out.
It is desirable to apply the principles of time assignment speech interpolation to systems in which a smaller number of transmission facilities are available. For example, it might be desirable to apply the TASI principle to private line communication systems having as few as four transmission facilities.
In U.S. Pat. No. 4,012,595 to Ota, an attempt has been made to reduce the number of transmission facilities required in a digital transmission system. The system disclosed by Ota comprises an encoder for converting the original speech signals into a digital form and a digital speech memory for temporarily storing these speech signals in order of their occurrence. The speech is then retrieved from the speech memory and transmitted in digital form. The storage of the speech is very short term and is an incident of the switching method used to place speech samples into transmission times on the system.
One problem associated with the system disclosed in the aforementioned Ota patent is that the transmission of speech signals in digital form requires a relatively wide bandwidth, thus lowering the efficiency of the system. While the Ota system is claimed to be effective for use with as few as 15 transmission channels, it would be desirable to provide a system for use with private line telephone systems which may have, for example, as few as four transmission facilities.
Efforts have been made to reduce the memory capacity required in TASI systems. It is an object of U.S. Pat. No. 3,644,680 to Amano et al to keep the memory capacity in a TASI system at a minimum. Accordingly, Amano et al stores only those speech signals to which facility channels have been assigned. Amano therefore keeps memory capacity to a minimum but fails to address the problem of freeze-out.
Accordingly, it is an object of the present invention to provide a telephone communication system which may interconnect a relatively large number of callers to a large number of listeners through a relatively small number of transmission lines.
It is a further object of the present invention to provide a telephone communication system in which freeze-out is maintained at a minimum despite a relatively low number of transmission facilities.